an al qaeda internet site provides a step by step manual in how to build a pressure cooker bomb. and this demonstration video from nepal news shows their potential power. >> we find the pressure cooker bombs are used quite extensive in afghanistan and pakistan. so in other words, this is a very similar device to the ieds that we have found throughout afghanistan used to kill our soldiers. >> law enforcement officials say there is still no solid evidence that links the marathon bombs to al qaeda or to anyone else. but tuesday, there was a new plea for help in trying to identify the bomber. >> any individual who expressed a desire to target the marathon, suspicious interest in researching how to create explosive devices, the noise of explosions in remote areas prior to yesterday which may have been used as tests by those responsible for these acts. someone who appeared to be carrying a unusually heavy dark colored bag yesterday around the time of the blasts and in the vicinity of the blasts. >> reporter: the efforts to learn who is behind the bombing often require great patience an